The present invention relates to an air-fuel ratio control system provided with an electrically-controlled carburetor in an intake system of an internal combustion engine, with a three-way catalytic converter in an exhaust system, and with an O.sub.2 -sensor for detecting the oxygen concentration of exhaust gases.
The air-fuel ratio control system controls the air-fuel ratio of the air-fuel mixture to a stoichiometric air-fuel ratio at which the three-way converter operates most effectively. However, it is desirable to vary the air-fuel ratio according to conditions of engine operation. For example, at rapid acceleration, the air-fuel mixture is enriched, and at deceleration, the mixture is diluted.
A control system for controlling the air-fuel ratio to a rich value and a lean value is disclosed in Japanese Laid Open No. 51-136035. The control system comprises a comparator for comparing the output of the O.sub.2 -sensor with a reference value, a proportion and integration circuit for amplifying and integrating the output of the comparator, a pulse generating circuit responsive to the output of the proportion and integration (hereinafter called PI) circuit for producing pulses, and a driver for driving an electromagnetic valve in dependency on the pulses for controlling the air-fuel ratio. In the system, the reference value is shifted according to conditions of engine operation so as to vary the air-fuel ratio. However, in such a system, since the proportion component in the output of the PI circuit is also varied by the shifting of the reference value, it is difficult to finely control the air-fuel ratio.
To resolve such a problem, a system for controlling the duty ratio of the pulses by delaying the integration component in the output of the PI circuit is disclosed in Japanese Laid Open No. 55-37560. In the system, when the integration component is delayed, the duty ratio of the pulses is decreased, so that the amount of air decreases to enrich the mixture. However, in the air-fuel ratio control system, the output voltage of the O.sub.2 -sensor fluctuates periodically, and the period of the fluctuation increases with decrease of the speed of the engine. If the delay time is constant, the duty ratio of the pulses varies with the variation of the period, that is the engine speed. Accordingly, a proper control of the air-fuel ratio can not be performed.